This invention relates to methods for the characterization of geological formation traversed by a borehole.
Resistivity measurements are at the origin of the logging services for the oil and gas exploration. One of the reasons which made these measurements so successful was the introduction of Archie's law, which allowed calculating the water saturation (Sw) of porous rock as a function of the measured resistivity (Rt). From there one calculates the hydrocarbon saturation (oil and/or gas), which is simply the complement to 1 for the water saturation, and this leads to the estimation of the total amount of oil in the reservoir by taking into account the measured porosity (φ) and the estimated volume of the reservoir (V). This estimation is thus given by equation: (1−Sw)φV. These parameters are of highest interest when seeking for giving the best estimation of the formation hydrocarbons production capacity.
Archie's law, which can be expressed by the following equation: Rt=Rw/Swn/φm, proved to be accurate in clean sandstone formations all around the world, and in general in most water-wet porous rocks, with both ‘n’ and ‘m’ exponent estimated to be at the value 2. This stability of the exponent values allowed to quickly make accurate evaluations of oil reserves for most sandstone reservoirs directly from porosity and resistivity logs. This technique was much less costly than the previously required extensive coring and core analysis campaigns, and was quickly adopted widely by the oil and gas industry as a standard petrophysical evaluation method.
With carbonate formations however, this turned out not to be satisfactory for most reservoirs. The values of the exponents ‘n’ and ‘m’ had to be adjusted using measurements on core samples, in order to fit resistivity measurements to the water saturation observed on cores. Typically the ‘m’ exponent remained close to 2 or slightly less than 2 (usually between 1.7 and 2), while the ‘n’ exponent could vary in a wide range of values from 2, or slightly less than 2, up to more than 5 (Values of 10 or more have been observed in laboratory experiments). This would have been fine if one could have established a correlation between the values of the exponents and the lithological nature of the rock layers, but no such general correlation could be established which meant that a given set of exponents validated for a given carbonate reservoir, or even a particular zone in the reservoir, could not be extrapolated to other reservoirs, or even to other zones within the same reservoir. The direct consequence of this lack of stability of the exponent values, is that one couldn't use the resistivity measurements from the logs taken in different wells drilled in the reservoir and apply one conversion law to calculate water saturations to estimate oil and gas reserves.
Furthermore, even in a given well the stability of the exponents is not guaranteed in advance in carbonate formations and it can be expected that ‘n’ will vary versus depth in the well.
Accurate oil reserve estimations in carbonates cannot be derived just from resistivity and porosity measurements but require either extensive coring and/or formation fluids sampling and/or independent log measurements of the water saturation. Unlike resistivity measurements which can be made quite deep into the formations (depth of investigation of several feet), the other known methods to measure water saturation are all shallow (a few inches) and therefore highly affected by mud invasion. This is why these direct measurements of water saturation cannot be considered representative of the true water saturation of the reservoir, which implies that extensive coring, and/or formation fluids sampling must be made in carbonates to make oil and gas reserve calculations. In fact, even the method based on coring and/or formation fluids sampling are questionable since cores properties are also affected by invasion, and fluid sampling does not provide a direct measurement of oil in place.
Many methods based on logging measurements other than coring and fluid sampling have been proposed to make better petrophysical evaluation of carbonates but all these methods rely on the classical formulation of Archie's law and are affected by the lack of stability of the ‘n’ exponent.